Superionic Water: The Strange Fluid Inside Uranus and Neptune

The substance that makes up the majority of our universe’s "ice" is not the clear, cold, brittle solid that clinks in a glass of lemonade. Deep inside the frozen behemoths of our outer solar system—Uranus and Neptune—lies a material so alien to our earthly intuition that it paradoxically combines the properties of a solid and a liquid, burns hotter than the surface of the sun, and is as black as charcoal.

This is superionic water.

For decades, it was a ghost in the machine of physics—a theoretical prediction that existed only in the lines of computer code and the scratching of chalk on university blackboards. But in a series of groundbreaking experiments conducted between 2018 and 2026, humanity finally crushed water hard enough to reveal its dark secret. We have confirmed the existence of a new state of matter, one that redefines our understanding of planetary interiors, magnetic fields, and the very nature of the most common molecule in the cosmos.

Part I: The Chameleon Molecule

To understand superionic water, one must first unlearn the simplicity of water as we know it. On Earth, water is the "universal solvent," a playful molecule of two hydrogen atoms clinging to a single oxygen atom (H₂O). We know it in three phases: the liquid that fills our oceans, the vapor that clouds our skies, and the solid ice that caps our poles.

However, water is a shapeshifter. It is one of the most polymorphic substances known to science. When subjected to pressures beyond the standard atmospheric pressure of Earth (1 atmosphere), water does not merely freeze; it transforms into a zoo of exotic crystalline structures.

If you squeeze water in a diamond anvil, you don't just get "harder" ice. You get entirely different architectures.

Ice I: The hexagonal lattice we know as snow.
Ice II, III, V, VI: Denser forms where the bond angles twist and contort to save space.
Ice VII: A cubic structure created at pressures exceeding 30,000 atmospheres (3 GPa), where the water molecules are packed so tightly they interpenetrate, yet remain locked as distinct H₂O units.
Ice X: At even higher pressures (around 700,000 atmospheres), the distinction between the hydrogen donor and acceptor blurs. The hydrogen atom sits exactly halfway between two oxygen atoms.

But physicists predicted that if you pushed even further—beyond 2 million atmospheres (200 GPa) and heated the sample to thousands of degrees—something fundamental would break. The water molecule itself would shatter.

The Theoretical Prediction

In 1988, a group of Italian physicists led by Pierfranco Demontis ran a computer simulation that spat out a bizarre result. They found that under extreme conditions, the oxygen atoms would remain frozen in a rigid crystal lattice (acting like a solid), but the hydrogen atoms would break their chemical bonds, ionize into protons, and flow freely through the oxygen cage (acting like a liquid).

They called it superionic water.

In this state, water is technically a solid because the oxygen lattice provides structural integrity. Yet, it is also a liquid because the protons flow like a fluid. It is also a conductor. Unlike pure water, which is an electrical insulator, superionic water conducts electricity—not via electrons, as in a copper wire, but via protons. It is a "fast proton conductor."

For thirty years, this remained a "computer hallucination"—a quirk of mathematics that no one could prove existed in the real world.

Part II: The Lightning and the Anvil

Proving the existence of superionic water required recreating the hellish conditions inside the mantle of Neptune: pressures of 2 to 4 million atmospheres and temperatures between 3,000°F and 8,000°F.

There is no container on Earth that can hold such material; the material itself is the container. To study it, scientists had to turn to the most powerful lasers on the planet.

The Omega Experiments (2018)

The first crack in the mystery came from the Omega Laser Facility at the University of Rochester. A team led by Marius Millot from Lawrence Livermore National Laboratory (LLNL) devised an experiment of brutal elegance.

They placed a thin layer of liquid water between two diamond plates. They then blasted the diamonds with UV laser pulses. The laser didn't heat the water directly; it vaporized the surface of the diamond, creating a rocket-like effect that drove a shockwave through the water sample. By carefully timing a sequence of shocks, they compressed the water to 2.5 million times Earth's atmospheric pressure.

For a few billionths of a second (nanoseconds), the water transformed.

Using optical pyrometry and velocimetry, the team observed that the water suddenly became opaque and conductive. It was behaving exactly as the 1988 simulations predicted. It had turned into superionic ice. This phase was officially cataloged as Ice XVIII.

The Black Ice Confirmation (2019-2021)

While the 2018 experiment proved the conductivity, it couldn't confirm the crystal structure. Was it really a solid oxygen lattice? Or just a dense plasma?

In 2019, Millot and his team moved to the National Ignition Facility (NIF)—the world's largest laser. They repeated the shock compression but this time fired X-rays at the tiny sample during the nanosecond it existed. The X-rays diffracted, creating a pattern that could only come from a crystalline solid.

The diffraction pattern revealed a Face-Centered Cubic (FCC) lattice of oxygen atoms. The oxygen was solid. The hydrogen was fluid. It was undeniably superionic.

But the visual appearance was the most shocking. In 2021, a team at the Advanced Photon Source (Argonne National Laboratory), led by Vitali Prakapenka, managed to create stable superionic ice not by shockwaves, but by static compression in a diamond anvil cell heated by lasers. They could look at it for longer than a nanosecond.

They found that superionic water interacts with light in a peculiar way. Because the protons are zooming around like a chaotic fluid, they interact with photons, closing the "band gap" of the material.

Result: Superionic water is black.

It absorbs visible light. If you could look inside Uranus, you wouldn't see a clear blue ocean; you would see a mantle of hot, charcoal-black ice.

Part III: The Messy Reality (2024-2026 Discoveries)

As of 2026, the story has evolved. Science rarely yields a perfect, simple picture, and superionic water is no exception.

Early models suggested the oxygen lattice was a perfect "Face-Centered Cubic" (FCC) structure—think of a cube with an atom at each corner and one in the center of each face. However, new high-precision X-ray laser experiments conducted at the European XFEL and SLAC in late 2024 and 2025 have revealed that nature is messier than we thought.

The latest findings show that the oxygen lattice in superionic water is not a perfect crystal. It is a "hybrid, misstructured sequence." It constantly shifts between Face-Centered Cubic (FCC) and Hexagonal Close-Packed (HCP) stacking.

Imagine trying to stack oranges in a crate. You can stack them in a square grid or a hexagonal honeycomb. Superionic water seems to be undecided, switching between these stacking modes chaotically. This "stacking disorder" might be crucial. It suggests that the material is even more fluid-like and dynamic than previously thought, potentially lowering the viscosity of the planetary mantles it inhabits.

This messiness means that the "ice" inside Neptune flows. It is not a rigid rock; it is a convecting, moving sludge of black crystal that transports heat from the core to the surface.

Part IV: The Mystery of the Ice Giants

Why does any of this matter? Because superionic water solves a 40-year-old mystery that has plagued astronomers since the Voyager 2 flyby.

In 1986 and 1989, NASA's Voyager 2 probe swept past Uranus and Neptune. It sent back magnetic data that confused everyone.

Earth: Our magnetic field is a nice, neat dipole (North and South) aligned roughly with our rotation axis. It is generated by the dynamo of molten iron in our core.
Jupiter/Saturn: Similar to Earth, but bigger.
Uranus/Neptune: Their magnetic fields are a disaster.

They are tilted wildly (Uranus is tilted 59° from its rotation axis).

They are offset from the center of the planet (by about 30% of the planetary radius).

They are "multipolar"—instead of just a North and South pole, they have complex, quadruple lobes.

A standard iron-core dynamo could not explain this. If the field were generated in the core, it would center itself. The fact that the field is off-center suggested it was being generated higher up*, in the mantle.

The Superionic Solution:

Superionic water fits this puzzle piece perfectly.

Conductivity: We know superionic water conducts electricity via protons. This makes it a fluid capable of generating a magnetic field (a dynamo).
Location: The pressures required to create superionic water (200 GPa+) exist in the "mantle" layer of Uranus and Neptune, about a third of the way down.
Nature: Because superionic water is a viscous, slushy solid-liquid hybrid, its convection patterns are different from liquid iron. It flows more slowly, and perhaps in thinner layers.

Simulations show that a dynamo generated in a thin shell of conductive superionic water—wrapping around a non-conductive rock core—naturally produces the tilted, off-center, multipolar magnetic fields Voyager saw. The "hot black ice" is the engine of the Ice Giants.

Part V: The Layering Controversy

However, science is a debate, not a monologue. A competing (or perhaps complementary) theory emerged around 2024, proposed by researchers like Burkhard Militzer at UC Berkeley.

This theory suggests that we might be looking at Immiscible Fluids.

As hydrogen and oxygen are squeezed inside these planets, they might separate. The hydrogen (being light) gets squeezed out of the mix, floating upward, while the carbon, nitrogen, and oxygen sink.

This could create a "stratified" interior where layers do not mix—like oil and vinegar.

Top Layer: Water-rich (conducting).
Bottom Layer: Hydrocarbon-rich (insulating or polymeric).

If the interior is stratified, convection stops. You can't have a giant overturning loop of material if the layers are chemically separated.

The current consensus (as of 2026) is trying to merge these ideas. It is likely that Uranus and Neptune possess a "Superionic Mantle" that is indeed conductive, but it might be sandwiched between a gaseous envelope and a deeper, stratified layer of hydrocarbon "diamond rain" and polymeric goo.

This complexity explains why Uranus and Neptune are so hard to model: they are not just balls of gas. They are chemical factories operating at millions of atmospheres.

Part VI: Implications for the Galaxy

The discovery of superionic water changes our map of the galaxy.

Astronomers have discovered thousands of exoplanets. The most common type of planet found so far is the "Mini-Neptune"—planets larger than Earth but smaller than Neptune.

Many of these are likely "Water Worlds"—planets made of 50% or more water by mass.

If superionic ice forms at the pressures found in Uranus (14 Earth masses), it certainly forms in these Super-Earths and Mini-Neptunes.

This means that Superionic Ice might be the most common form of water in the universe.

Liquid water (the stuff of life) might be a rare surface veneer, a thin skin on a chaotic universe of hot, black, proton-conducting crystal.

Implications for Life:

Can life exist in superionic water? Almost certainly not. The bonds break; the protons flow; the temperatures are thousands of degrees.

However, the presence of a superionic mantle generates a magnetic field. And a magnetic field is a shield. It protects a planet's atmosphere from being stripped away by stellar winds.

Therefore, the superionic ice deep inside a Water World might be the silent guardian that allows a thin layer of liquid, habitable ocean to exist on the surface. Without the hot black ice below, the liquid blue water above might be lost to space.

Epilogue: The New Phase

We used to think of water as simple. H-O-H. It freezes at 0°C; it boils at 100°C.

We were wrong.

Water is a monster.

Squeeze it, and it turns into a variety of crystals (Ice I through XVII).

Squeeze it harder, and it turns into a glowing, black, electricity-conducting hybrid of solid and liquid (Ice XVIII - Superionic).

Squeeze it even harder (Ice XX?), and it might become metallic.

The study of superionic water is a testament to human ingenuity. We took a substance that exists only in the hearts of distant giants, simulated it with math, and then, briefly, brilliantly, brought it to life with the most powerful lasers ever built. We held a piece of Neptune in a laboratory in Rochester, New York, and watched it turn black.

As we look toward the 2030s and the proposed Uranus Orbiter and Probe (UOP) mission, we are no longer flying blind. We know what lies beneath the cyan clouds. We know that the heart of the Ice Giant is not frozen still, but alive with the flow of protons through a lattice of dark crystal.

The universe is strange. And the water in your glass is the strangest thing of all.